Actuating spindles in potentiometers and variable resistors



Sept. 1950 E. E. WEBSTER ET AL 2,952,826

ACTUATING' SPINDLES IN POTENTIOMETERS AND VARIABLE RESISTORS Filed May 6, 1959 2 Sheets-Sheet l P 1960 E. E. WEBSTER ET AL 2,952,826

ACTUATING SPINDLES IN POTENTIOMEITERS AND VARIABLE RESISTORS Filed May 6, 1959 2 Sheets-Sheet 2 Fla. 4.

United States Pa en ACTUATIN G SPIN DLES IN POTENTIOMETERS AND VARIABLE RESISTORS Edmund Ernest Webster, Ilford, and John Clark, Swindon, England, assignors to The Plessey Company Limited, London, England, a British company This invention relates to actuating spindles in potentiometers and variable resistors. Hitherto such spindles have normally been made from round bar material, one end of the bar being machined to produce a flat at one side of the spindle for engagement by the actuating knob, and a plate is normally attached to the other end, for example by riveting onto a reduced-diarneter part of the spindle, to serve as a stop and as a support for a brush carrier and, where desired, as a switch actuator.

The present invention has for an object to provide an improved spindle adapted to take the place of the known spindle, which can be manufactured economically. According to the invention the spindle is formed by bending a strip of suitable material, for example of halfhard brass, along longitudinal lines to a substantially triangular cross-seotional profile, preferably with a small gap left between the opposed edges of the strip in one of the sides of the triangle, the corners of the triangle being preferably rounded, so that the triangular shaft thus obtained will bear smoothly with substantially a three-point location, in a round bush, with which it is in contact at the rounded edges of the shaft; the resilience of the material can be used to ensure this contact and compensate for wear. Each side of the triangle can also be used as a flat for location of the operating knob, thus avoiding the need for producing such flat by machining, while again the resilience of the bent material can be utilised to ensure tight co-operation of this fiat with the complementary flat surface of the knob. This avoids the necessity of providing a spring steel clip in the knob as hitherto customary. A stop arm and, if desired, a switch actuating lug can be readily obtained by bending over integral extension pieces at the end of the shaft remote from the knob; the blanking away of the material of the extension apart from the required strips may be effected either after or preferably before the strip has been bent to the triangular profile. While in the hitherto usual construction, a stop member riveted to the end of the spindle supports and operates the brush carrier with the interposition of a blanked sheet of insulating material, an arrangement which is liable to considerable back-lash, it is preferred in connection with the spindle according to the present invention to support and operate the brush carrier by means of a flanged bush having a triangular bore fitting with an interference fit over the triangular spindle, so as to resiliently deform the spindle profile, one end of said bush being supported against the bentover stop arm, while its other end bears against the bearing bush in which the spindle is rotatable. Axial location of the shaft in the bush against undesired withdrawal can be obtained by forming a pip in the corner opposite the gap in the triangle, either after insertion of the shaft in the bush or, if preferred, by arranging an axial groove in the bush, along which the pip may be introduced when the spindle is turned into a position which it is prevented by the stop member from as- 2,952,826 Patented Sept. 13, 1960 ice suming during the operation of the potentiometer. The invention is illustrated in the drawings.

Figure 1 is an enlarged end view of one form of spindle according to the invention,

Figure 2 is a side elevation thereof, and

Figure 3 is an axial section of one form of potentiometer incorporating a spindle such as that shown in Figures l and 2.

Figure 4 similarly shows a modification, and Figure 5 is an end view of the modified actuating member.

Referring first to Figures 1 and 2, the spindle is formed from a flat strip of material 1 by bending its cross-section about three lines extending longitudinally of the strip to form an equilateral triangle having rounded corners 2, 3 and 4, with a gap between the two opposed edges of the strip in the side of the triangle opposite to the corner 2. A spindle for use in a A diameter bore may for example be made of half-hard brass of 25 S.W.G. (i.e. of a thickness of 0.2 mm.). Two of the fiat sides of the shaft are, at one end of the shaft, formed with integral longitudinal extensions 5 and 6 respectively, which are bent outwardly by right angles, whereby the extension 5 forms a radially extending stop arm, while the extension 6 is cranked by two further bends to form a switch actuating lug.

Figure 3 illustrates the use of a spindle according to the invention in a potentiometer or variable resistor. The potentiometer illustrated in Figure 3 comprises an insulating base 7 in which is embedded a resistor track 8 and a contact ring 9, the latter being integral with a terminal lug 10. The base 7 is mounted in a housing body 11, for example of a light metal alloy, having an integral spindle bush 12 through which the potentiometer spindle 13, constructed as illustrated in Figures 1 and 2, extends, with its rounded edges 2, 3 and 4 resiliently bearing on the inner surface of the bush. Attached to that end of the spindle 13 which carries the stop arm and the switch actuating member 6 is an actuating body formed as a flanged bush 14 of polystyrene or other insulating synthetic-resin material, which has a triangular central bore fitting tightly over the spindle 13, and which supports and guides a brush-holder member 15 of resilient metal. The latter is equipped with two brushes 16 and 17 for respective co-operation with the contact ring 9 and the potentiometer track 8. A pip 20 is formed in the edge 2 of the shaft 13 which, when the potentiometer is assembled as shown, bears against the outer surface of the bush 12 so as to hold the shaft against movement to the right (as seen in Figure 3), thereby providing resilient application pressure for the brushes 16, 17. In order to allow this pip to be formed before assembly of the device, the bush 12 is provided with a longitudinal internal groove 21 at a position which is so chosen that the operation of the stop 5 or other end stop will prevent the pip 20 from returning into alignment with the groove 21 once the potentiometer has been assembled and returned from the position of alignment. A dust cap 22 encloses the potentiometer mechanism and may if desired carry an on/otf switch for operation by the lug 6.

While the invention has been described with reference to one particular form of potentiometer, it will be readily evident to those skilled in the art that its use is by no means limited to this particular form of potentiometer, and that the various details of the spindle described with reference to the drawings may be modified according to requirements. Thus the part intended for insertion into the knob may be slightly opened to obtain a tighter fit in the knob.

In the embodiment illustrated in Figure 4 the actuating member 13 has been replaced by a modified actuating 3 member 24, of which an end view is shown in Figure 5. The actuating member is preferably moulded of polystyrene and integnally embodies a stop lug 27 and a switch actuating lug 25; itv also differs from the actuating member of Figures 1 to 3 by the fact that the locating slot for the brushholder member 15 is covered by a' layer 26 of insulating material, against which the bend 15a of the brush-holder member 15 is supported in fulcrum fashion. The bent-over arms 5a of the spindle 13 are. not required to act as end stops for the movement of the spindle and need not therefore project beyond the periphery of the flange of the member 24, their only function being to prevent this member from being pulled on the spindle. Figure 5 clearly shows the triangular opening 28 which accommodates the triangular profile offlthe spindle, thus forcing the actuating member 24 to" rotate with the spindle without any lost motion.

What we claim is:

I. In a rotary-type variable resistor device, the combination comprising a shaft of triangular cross sectional profile, a bush in Which said shaft is mounted for rotation with sliding contact between said bush and shaft along'three linear zones extending longitudinally of the bush and shaft, and a resistance-varying contact member connected to the shaft for common movement about the axis of said bore.

2. A combination as claimed in claim 1, wherein the shaft consists of a strip of. metal which is bent along longitudinal lines to a substantially triangular cross-sectional profile.

3. A spindle as claimed in claim 2, wherein a small gap is left between the opposed edges of the bent strip in one of the sides of the triangle.

4. A spindle as claimed in claim 3, wherein the corners ofvthe triangular profile are so shaped as to provide smooth bearing surfaces for joint co-operation with the bore.

5. A variable resistor device as claimed in claim 3 wherein the bush has a bore slightly smaller in diameter than the diameter of a circle passing through the corners of the shaft when the latter is in its unstressed condition.

6. A variable resistor device as claimed in claim 5, which further comprises an actuating knob provided with a bore having at least one fiat side for co-operation with one of the sides of the triangular cross-section of the shaft.

7. A variable resistor device as claimed in claim 5 further comprising a switch-actuating member of insulating material having a non-circular bore fitting tightly over the triangular profile of the shaft for common rotation therewith and having a flange shaped for supporting and locking engagement with a brush holder.

8. A variable resistor device as claimed in claim 7, wherein at least one side of the shaft is formed with an extension bent over at the back of the actuating member'to preventthe latter from being withdrawn.

9. A variable resistor device as claimed in claim 7, wherein the actuating member consists of moulded synthetic resin material.

10. A variable resistor device as claimed in claim 7, wherein the actuating member consists of moulded polystyrene.

11. A variable resistor device as claimed in claim 7, wherein the actuating member is formed with an integral switch-actuating lug.

12. A variable resistor device as claimed in claim 8, wherein the bearing bush is held substantially in engagement with the opposite side of the actuating member by a portion of the shaft which is deformed to project radially from one of the corners of the triangular profile.

13. A variable resistor device as claimed in claim 12, wherein the bearing bush is formed with a longitudinal groove through which the said projecting portion can pass when in alignment therewith, stop means being provided for holding the bush after assembly in a position which the projection is prevented by a stop from reaching moperation.

14. A resistor device as claimed in claim 12, wherein the projection is formed in the edge opposite to the gap left between the edges of the strip.

References Cited in the file of this patent UNITED STATES PATENTS Re. 24,046 Nathan July 26, 1955 1,420,042 Kritz June 20, 1922 1,784,436 Hadley Dec. 9, 1930 2,441,265 Frye May 11, 1948 2,614,405 Clausen Oct. 21, 1952 FOREIGN PATENTS 496,459 Great Britain Nov. 30, 1938 

